In a typical cellular network, also referred to as a wireless communication system or a communications system, a User Equipment (UE), communicates via a Radio Access Network (RAN) to one or more Core Networks (CNs).
A user equipment is a device that may access services offered by an operator's core network and services outside the operator's network to which the operator's radio access network and core network provide access. The user equipment may be any device, mobile or stationary, enabled to communicate over a radio channel in a communications network, for instance but not limited to e.g. mobile phone, smart phone, tablet computer, sensors, meters, vehicles, household appliances, medical appliances, media players, cameras, Machine to Machine (M2M) device, or any type of consumer electronic, for instance but not limited to television, radio, lighting arrangements, tablet computer, laptop or Personal Computer (PC). The user equipment may be portable, pocket storable, hand held, computer comprised or vehicle mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity.
The user equipment is enabled to communicate wirelessly in the communications system. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, comprised within the communications system.
The radio access network covers a geographical area which is divided into cell areas. Each cell area is served by a base station. In some radio access networks, the base station is also called e.g. Radio Base Station (RBS), evolved NodeB (eNB), NodeB or B node. A cell is a geographical area where radio coverage is provided by the base station at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base station communicates over an air interface operating on radio frequencies with the user equipment within range of the base station.
Standardised by the third Generation Partnership Project (3GPP), High Speed Downlink Packet Access (HSPA) supports the provision of voice services in combination with mobile broadband data services. HSPA comprises High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA) and HSPA+. HSDPA allows communication systems based on the Universal Mobile Telecommunications System (UMTS) to have higher data transfer speeds and capacity. In HSDPA, a new transport layer channel, High Speed-Downlink Shared CHannel (HS-DSCH), has been added to the UMTS release 5 and further specifications. It is implemented by introducing three new physical layer channels: High Speed-Shared Control Channel (HS-SCCH), Uplink High-Speed Dedicated Physical Control Channel (HS-DPCCH) and High Speed-Physical Downlink Shared Channel (HS-PDSCH). The HS-SCCH informs the user equipment that data will be sent on the HS-DSCH, two slots ahead. The HS-DPCCH carries acknowledgment information and a current Channel Quality Indicator (CQI) of the user equipment. This CQI value is then used by the base station to calculate the amount of data that should be sent to the user equipment in the next transmission. The HS-PDSCH is the channel mapped to the above HS-DSCH transport channel that carries actual user data. HSPA may recover fast from errors by using Hybrid Automatic Repeat reQuest (HARQ). HARQ is a technique that enables faster recovery from errors in communications systems by storing corrupted packets in the receiving device rather than discarding them. Even if retransmitted packets have errors, a good packet may be derived from the combination of bad ones.
Multiple Input Multiple Output (MIMO) refers to any communications system with multiple antennas at the transmitter and/or the receiver, and it is used to improve communication performance. The terms input and output refer to the radio channel carrying the signal, not to the devices having antennas. At the transmitter (Tx), multiple antennas may be used to mitigate the effects of fading via transmit diversity and to increase throughput via spatial division multiple access. At the receiver (Rx), multiple antennas may be used for receiver combining which provides diversity and for combining gains. If multiple antennas are available at both the transmitter and receiver, then different data streams may be transmitted from each antenna with each data stream carrying different information but using the same frequency resources. For example, using two transmit antennas, one may transmit two separate data streams. At the receiver, multiple antennas are required to demodulate the data streams based on their spatial characteristics. In general, the required minimum number of receiver antennas is equal to the number of separate data streams. 4×4 MIMO, also referred to as four branch MIMO, may support up to four data streams. In general, MIMO may be n×n MIMO, where n is the number of antennas and is positive integer. For example 2×2 MIMO, 8×8 MIMO, 16×16 MIMO etc.
Some terms will now be explained. A transport block holds the data that is going to be transmitted, and the transport block is converted into a codeword. A codeword may be defined as the number of transport blocks which have the same HARQ-process identifier. A codeword may be mapped to a number of layers. The term “layer” is synonymous with “stream.” For MIMO, at least two layers must be used. The number of layers is always less than or equal to the number of antennas. Precoding modifies the layer signals before transmission. A transmission rank refers to the number of transmitted data stream.
Several new features are added for the long term HSPA evolution in order to meet the requirements set by the International Mobile Telecommunications-Advanced (IMT-A). The main objective of these new features is to increase the average spectral efficiency. Spectral efficiency is a measure of how efficiently a limited frequency spectrum is utilized. It refers to an information rate that may be transmitted over a given bandwidth in a specific communications system. One possible technique for improving downlink spectral efficiency may be to introduce support for four branch MIMO, i.e. to utilize up to four transmit and receive antennas to enhance the spatial multiplexing gains and to offer improved beam forming capabilities. Four branch MIMO provides up to 84 Mbps per 5 MHz carrier for high Signal to Noise Ratio (SNR) user equipments and improves the coverage for low SNR user equipments.
Spatial multiplexing mentioned above is a transmission technique in MIMO to transmit independently and separately encoded data signals, so-called data streams, from each of the multiple transmit antennas. Therefore, the space dimension is reused, or multiplexed, more than one time. If the transmitter has N_t antennas and the receiver has N_r antennas, the maximum spatial multiplexing order (the number of data streams) is:N_s=min(N_t,N_r)
This means that N_s number of data streams may be transmitted in parallel, ideally leading to an N_s increase of the spectral efficiency (the number of bits per second and per Hz that may be transmitted over the wireless channel).
Channel feedback information, also referred to as CSI, enables a scheduler to decide which user equipments that should be served in parallel. The user equipment is configured to send at least one of the following three types of channel feedback information: a CQI, a Rank Indicator (RI) and a Pre-coding Matric Indicator (PMI). CQI is an important part of channel information feedback. The CQI provides the base station with information about link adaptation parameters which the user equipment supports at the time. The CQI is utilized to determine the coding rate and modulation alphabet, as well as the number of spatially multiplexed data streams. RI is the user equipment recommendation for the number of layers, i.e. the number of data streams to be used in spatial multiplexing. RI is only reported when the user equipment operates in MIMO mode with spatial multiplexing. The RI may have the values 1 or 2 in a 2×2 MIMO configuration i.e. one or two transmitted data streams. The RI may have the values from 1 and up to 4 in a 4×4 MIMO configuration. The RI is associated with a CQI report. This means that the CQI is calculated assuming a particular RI value. The RI typically varies more slowly than the CQI. PMI provides information about a preferred pre-coding matrix in a codebook based pre-coding. PMI is only reported when the user equipment operates in MIMO mode. The number of pre-coding matrices in the codebook is dependent on the number of antenna ports on the base station. For example, four antenna ports enables up to 64 matrices dependent on the RI and the user equipment capability. A Precoding Control Indicator (PCI) indicates a specific pre-coding vector that is applied to the transmit signal at the base station.
Introduction of four branch MIMO will require a new feedback channel structure to send the CQI and PCI information to the base station. To reduce the signalling overhead at the downlink and the uplink, it was recommended to use two codewords for four branch MIMO. For designing uplink signalling channel, i.e. HS-DPCCH, it was agreed to use a similar structure as that of two antenna MIMO, described in 3GPP release 7. When reporting CQI, RI and PCI, the CSI may be reported in two reporting intervals. This structure is attractive in terms that it requires minimal standards change. The performance with this structure is very close to that of ideal reporting. In general, the base station needs to wait for two reporting intervals to schedule the user equipment for data transmission. If the reporting period is configured to a high value, say for example 8 msec, the base station needs to wait 16 msec to schedule the user equipment. For a high speed user equipment, this introduces delay and the performance degradation is very severe.
An overview of channel quality reporting and base station procedures for two branch (2×2) MIMO (3GPP release 7 MIMO) will now be described with reference to FIG. 1. FIG. 1 shows the messages exchanged between base station 101 and the user equipment 105 during a typical data call set up. The method comprises the following steps, which steps may be performed in any suitable order:
Step 101
From the Common Pilot Indicator CHannel (CPICH), the user equipment 105 estimates the channel and computes the CQI and the PCI. The CPICH is a downlink channel broadcast by the base station with constant power and of a known bit sequence.
For two antennas, the CQI is computed as follows:
  CQI  =      {                                                                      15                ×                                  CQI                  1                                            +                              CQI                2                            +              31                                                                          CQI              S                                          ⁢                                                                                                                              when                      ⁢                                                                                          ⁢                      2                      ⁢                                                                                          ⁢                      transportblocks                      ⁢                                                                                          ⁢                      are                                        ⁢                                                                                                                                                                                  preferred                    ⁢                                                                                  ⁢                    by                    ⁢                                                                                  ⁢                    theuser                    ⁢                                                                                  ⁢                    equipment                                                                                                                                                                                                      when                      ⁢                                                                                          ⁢                      1                      ⁢                                                                                          ⁢                      transportblock                      ⁢                                                                                          ⁢                      is                                        ⁢                                                                                                                                                                                  preferred                    ⁢                                                                                  ⁢                    by                    ⁢                                                                                  ⁢                    theuser                    ⁢                                                                                  ⁢                    equipment                                                                                          
Where the CQI is the channel quality per individual layer. CQI1 is the CQI corresponding to the first layer CQI2 is the CQI corresponding to the second layer CQIS is the CQI for the single stream. 31 is the offset factor.
It can be observed from equation above that if the CQI is less than 31, the rank information is 1, otherwise the rank information is 2. The PCI is the precoding information bits selected in the subset of the codebook corresponding to the rank information.
Step 102
The information computed in step 101, i.e. the CQI and PCI, along with a HARQ ACK/NACK is reported to the base station 105 using the HS-DPCCH. The periodicity of HS-DPPCH is one sub-frame (e.g. 2 msec).
The structure of the HS-DPCCH is shown in FIG. 2a and FIG. 2b. FIG. 2a shows an example of how the PCI and the CQI are located in the structure. As well-known, the HS-DPCCH sub-frame structure comprises one slot for HARQ ACK/NACK transmissions and two slots for CQI/PCI transmissions. In the following, even though the text or the drawings may refer to a HARQ ACK, it is appreciated that this may also be a HARQ NACK.
The HS-DPCCH sub-frame structure in FIG. 2a for the TTI=2 ms comprises a field comprising a HARQ ACK or NACK. TTI is short for Transmission Time Interval. The HARQ ACK/NACK notifies the base station 105 whether or not the user equipment 101 has received the correct downlink data. The HARQ ACK/NACK field defines like this: 1-NACK, 0-ACK. The CQI reflects the PCI based on CPICH strength. Each sub-frame comprises a HARQ ACK/NACK, two CQI fields and one PCI field. In other words, every sub-frame comprises the same fields.
The HS-DPCCH in 3GPP release 5 to release 9 is based on a 1×SF256 solution. The structure of the HS-DPCCH is shown in FIG. 2b. As well-known, the HS-DPCCH sub-frame structure comprises one slot for HARQ ACK/NACK transmissions and two slots for CQI/PCI transmissions. This structure should also be used for four branch MIMO.
HARQ Details: For 3GPP release 7 MIMO the HARQ ACK/NACK codebook comprises six codewords plus the PRE/POST.
CQI/PCI Details: In 3GPP release 7 there are 5 or 2×4 bits allocated for describing the CQI depending on the CQI type. There are 30 or 15 CQI values per stream for rank 1 and rank 2, respectively, and the rank is implicitly signalled via the CQI. Furthermore, CQIs for each data stream are signalled independently of each other. In addition to the CQI bits there are two bits allocated for signalling the preferred pre-coding information. The 7 (or 10) information bits are then encoded into 20 channel bits that are transmitted during the second and third slot.
Returning to FIG. 1.
Step 103
Once the base station 101 receives the complete CQI, PCI and HARQ ACK/NACK, it allocates the required channelization codes, modulation and coding, precoding channel index to the user equipment 105 after scheduling.
Step 104
Information about the required channelization codes, modulation and coding, precoding channel index from step 103 is transmitted to the user equipment 105 using the HS-SCCH.
Step 105
When the user equipment 105 has received the information in step 105, the user equipment 105 detects the HS-SCCH.
Step 106
Once the user equipment 105 has detected the HS-SCCH, the downlink transmission starts through data traffic channel using the HS-PDSCH.
In general, HS-DPCCH design depends on many factors like number of codewords supported, number of HARQ processes, precoding codebook etc. Four branch MIMO should support two codewords and two HARQ processes.
The current HSDPA system (3GPP release 7-10) supports one or two transmit antennas at the base station 101. For these systems, from channel sounding, the user equipment 105 measures the channel and reports the channel state information in one sub-frame. A sub-frame may be defined as for example one TTI which may be e.g. 1 ms or 2 ms. Typically this channel state information report comprises the CQI which explicitly indicates the RI and the PCI. The user equipment sends this report periodically for every sub-frame, i.e. for every TTI to the base station. Once the base station receives this report it grants the Modulation and Coding Scheme (MCS), number of codes, rank and the PCI to each specific user equipment based on the scheduler metric. Based on this information, the base station may optimize the downlink throughput for each TTI.
Introduction of four branch MIMO will require a new feedback channel structure to send the CQI and PCI information to the base station 101. To reduce the signalling overhead at downlink and uplink, two codewords should be used for four branch MIMO. For designing uplink signalling channel (i.e. H-DPCCH), a similar structure that of two antenna MIMO (3GPP release 7) should be used. The structure for reporting CQI, RI and PCI is attractive in terms that it requires minimal standards change. However, this structure is not optimized for lower rank transmissions as for rank 1 and rank 2 because the CQI reported in the second reporting interval is redundant.